Sewing!!!!!!!!!!!!!!!!
Honestly........the quality is in the sewing machine (and yarn!) and then the operator. When it comes to seat covers, you will need fairly durable fabrics, with that you will need a fairly powerful machine, and an operator that can handle said machine! Door cards are fairly simple, but seats require expertise, thats why they are as expensive as they are. You may achieve a good appearance with a bog standard sewing machine, but it wont last because the yarn is not durable enough!
Door cards are simple enough with spray glue and any fabric, but stick to bought seat covers or upholstered covers to get that professiona, long lasting quality.
Andy